Parasitic oscillation suppression with compressed air condensers in thermionic circuits



Patented Mar. 9, 1943 PARASITIC OSCILLATION SUPPRESSION WITH COMPRESSED AIR CONDENSERS IN THERMONIC CIRCUITS James L.- Finch, Patchogue, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 2, 1939, Serial No. 287,934

This invention relates to thermionic circuits in connection with which compressed air tuning condensers are employed, and relates particularly to high frequency power amplifier circuits of a radio transmitter using compressed air condensers.

' An object of this invention is to discourage or entirely suppress any parasitic oscillations by connecting in series with the compressed air condensers a resistance shunted by an inductance.

Another object of this invention is to improve radio transmitting high frequency amplifier circuits generally. Compressed air condensers of the usual construction have a certain amount of inherent inductance in series with the capacity element, which inherent inductance is generally greater than that of the ordinary plate condenser, with the open air as a dielectric. This inherent inductance combines with other capacities of the circuit to produce a resonant circuit at a much higher frequency than that for which the circuit is designed. The presence of such a resonant circuit very often leads to parasitic oscillations at the frequency of these circuits. In the prior art, these compressed air condensers have been usually connected directly into the circuit in which the capacity which they provide is required. This has made these circuits subject to parasitic oscillation as mentioned above.

The single figure of the drawing shows a typical power amplifier of push-pull variety in connection with which compressed air tuning condensers are used.

In the drawing, I represents a source of oscillating power used for exciting the power amplifier circuit.- Itmes 2 and I2 are the thermionic triodes used in this amplifier, each having the usual anode, grid and cathode. these tubes are tuned with an inductance 2| and variable tuning condenser 22', as indicated. The grids are connected to a source of negative potential C through the mid-tap of 2| and through an impedance 23. The bias source is by-passed by a condenser 24. The plates of tubes 2 and i2 are tuned with a plate inductance 25 which in turn is shunted with tuning condensers 26 and 34. A source of plate power +13 is connected to the plates through the mid-tap of 25 and through an impedance 28. This source of plate power is usually by-passed to ground by a condenser 29. The usual circuit method of neutralization against parasitic oscillations due to regeneration through the plate to the grid capacity of the tubes 2 and I2 is provided by The grids of means of neutralizing condensers'3 and I3 of the compressed air type. these neutralizing condensers and the grids con-' tain a small amount of inherent circuit inductance 4 and I caused by the condenser construc-' tion and circuit leads which is usually of relatively small impedance at the operating frequency generated at source I and impressed on the grids by inductive coupling between its output inductance 30 and the grid inductance 2|.

Plate tuning condenser 26 is assumed to be of occupied. Due to the construction of such compressed air condensers they have a small amount of inherent inductance associated with one of the terminals which is represented by numeral 3 I. In the prior art, this condenser is generally connected directly to the plate of tube 2, omitting the elements 32 and 33 of this invention.

I have found that the inherent inductance in combination with the casing capacity of the condenser element 34. of the compressed air con-.

denser casing, and in combination with tuning condenser 26, and any stray capacity of the circuits, produces a resonant circuit at a frequency very much higher'than the normal frequency. In this case, inductance 2'5 simply serves as a choke through which anode potential is fed to the plates. At this parasitic frequency, the inductance 4 and M in series with the compressed air neutralizing condensersalso becomes important, the reactance 4 and I4 becoming of the same order as the reactance of 3 and I3. Thus, at this frequency the circuit is no longer neutralized and sufficient regeneration is introduced to cause oscillations.

By the circuit method of my invention, I connect a resistor 32 and an inductor 33 in the cir-" cult of the compressed air condenser 34, as

shown on the drawing. At the normal operating frequency, the resistance of 32 is much higher than the reactance of 33, and the reactance of either,,32 or-33 is very low compared with the reactance of 34. Thus, they have very little effect on the tuning of the circuit and absorb very little power from the circuit. At the para- The connection between sitic frequency, however, the reactance' f {l becomes very much higher and becomes oiiebhe 'same order of magnitude as the resistance 0112.

At this frequency, therefore, 32 absorbs power from the circuit and discourages or suppresses the parasitic oscillations.

Modifications of. myinvention can consist of,

the substitution of-elements 32 and. oiany;

other elements which have the characteristic of low loss and low reactance at the operating frequency and high lossat the parasitic frequency,

Further modifications will consi'stloi the use f which with other circuit elements provide paralof compressed air condensers between'the plates i of tubes 2 and I2 and ground or between the plates of tubes 2 and I2, and the-high voltage end of by-pass condenser 29. I a Still further modifications will consist oi any other type of thermionic circuits and tubes other and a compressed air dielectric condenser having high capacity per unit of space occupied, said condenser also having inherent inductance lel paths wherein voltages of frequencies other than said operating frequencies are set up to destroythe neutralization ,0! said circuits, and means for reducing the eiiect 0! said voltages including a resistance and an inductance in parallel connected in series with said condenser, the value 0! said resistance being high as compared tothereactancejoisaid last inductance at said :operating; frequency, the reactance of said resistanceandofsaid last inductance being low a high frequency push-pull amplifier ciras compared to the reactance of said condenser.

cuit, comprising a pair of electron discharge de- V vices, eachghaving anode, grid and cathode cirthan the triodes shown in'which compressed air condensers are employed and in which this method of discouraging parasitic oscillations is employed. Therefore, this invention should not be limited to; the precise circuit arrangementshown; Whatisclaimedisr In a high frequency wave energy system a circuit including an electron V discharge device coupled insai'd'circuityan anode circuit,' said anode circuit-having an inductance'and a capacity in parallel tunedto r'esona'nce at' the fre-' than the reactance ofjs'aidfirst named capacity.

2 Ina wave amplifying system, a pair oielec- 'trbn' discharge s'ys m having inputelectrodes and output electrodes connected in neutralized wave repeating circuits including meansior tuning at least one of said circuits to an operating 1 Irequency, said last means including inductance cuitsfa sourceof input voltage supply connected tothe grid circuit 01 said electron discharge devices, meansior tuning the grid circuit to the frequency or theinput voltage. three compressed air dielectric condensers, each one having inherent inductance and capacities in series with the condenser capacitive elements whichwmay reduce parasitic oscillations, means ior neutralizing each one of said grid circuits oi the electron' discharge devices including one of said compressed airdielectric condensers, means for tuning theanode circuit of said electron discharge devices including an inductance, a variable air dielectric condenser and the third one of said compressed air dielectric condensers, which is included in ashunt circuit across the anode circuit, said shunt circuit also including means to eliminate said parasitic oscillations which are caused by the inherent inductance v and capacitiesof said compressed air dielectric condensers, said means including an inductance and agresistance connected in parallel and in series with said'third compressed air dielectric condenser, the resistance being of a value higher than the reactance of. said inductance at the fre quency to which said grid circuit is tuned, the reactance of the resistance and the inductance being low' as compared to the reactance 0! said compressed air dielectric condensers.

' JAMES L. F'INCH. 

